My Volvo 850 has had a rather slow starter motor since I've owned it. But now it is getting really slow. The battery is relatively new. Time for a new/rebuilt starter motor?

The armature in your starter motor spins inside of two phosphur bronze bushes which have been impregnated with oil when they were fitted. That oil dries out and the bushes wear out. This means that the armature can spin off centre and strike the internal poles and pole shoes. This is called poling and WILL cause serious or irreparable damage to the starter motor. Yes, it's time to have the starter motor removed and overhauled or replaced. Also, have a look at the two main battery cables and have them replaced with double zero B & S cable as some Volvo's were fitted with small diameter cables which give troubles after a few years.

Check cables first. Take starter in and have it bench tested at large chain store (free).